Voltage Drop Calculator

How the Voltage Drop Calculator Works

This calculator uses the full R+X impedance method from NEC Chapter 9, Table 9 — the code's own table of effective conductor impedance, published separately for copper and aluminum, in steel (magnetic) and PVC (non-magnetic) conduit, for every conductor size from 14 AWG through 750 kcmil at 0.85 power factor test conditions. Most free voltage drop calculators skip that table entirely and just use a simplified 2R (resistance-only) formula — fine for low-power-factor, small-conductor circuits, but it quietly loses accuracy as conductor size and circuit reactance grow, which is exactly the regime larger commercial and industrial feeders live in.

For each conductor size, material and conduit type, Table 9 gives a resistance (R) and reactance (X) in ohms per 1,000 feet. This calculator scales both by the actual one-way run length, then applies the standard phasor voltage-drop formula: for a three-phase circuit, VD = √3 × I × (R·cosθ + X·sinθ) × L/1000; for single-phase, the multiplier is 2 instead of √3. The load's power factor (cosθ) and its derived reactive component (sinθ) are both taken directly from the Power Factor field, so a low-power-factor load — which draws more current for the same real power and drives more current through the conductor's reactance — is reflected correctly in the result, not glossed over.

Example Scenario

The calculator's default field values — 480 V, 100 A, a 150 ft one-way run at 0.85 power factor, on 3/0 AWG copper in steel conduit, three-phase — work out to a voltage drop of about 2.27 V, or 0.47% of system voltage: well inside both NEC informational thresholds. Changing conduit type from steel to PVC, or conductor material from copper to aluminum, changes only the R and X values pulled from Table 9 — everything else in the calculation stays the same, which is a fast way to see how much of a given design's margin is actually coming from conduit choice versus conductor size.

Why This Matters — NEC 215.2(A)

NEC 215.2(A) Informational Notes recommend keeping voltage drop to 3% or less on a branch circuit alone, and 5% or less for the combined feeder-plus-branch-circuit total. These are informational notes, not enforceable code requirements — a design can pass inspection without meeting them — but exceeding them is a real, practical problem: motors run hot and lose starting torque, electronic equipment and drives misbehave on undervoltage, and lighting output and lamp life both suffer. This calculator flags both thresholds directly on the result, using the actual color coding the rest of this site uses for a passing versus a marginal result.

Related Calculators

Voltage drop and conduit fill both come from NEC Chapter 9, and a feeder redesign usually needs both checked together — see the Conduit Fill calculator on the main calculator suite. For DC systems specifically, see the Battery DC Short-Circuit Current calculator and the Transformer Thermal Loading & Insulation Life calculator. The full suite of eleven free calculators — arc flash, cable tray, battery sizing, and more — is available from the powerengcalc.com home page.

For engineering consulting, feeder and distribution system design, or forensic investigations, visit pefgconsulting.com.